The invention pertains to the art of the the electrically actuated vent or flue dampers used to save energy in domestic furnaces and the like.
There are a number of different types of electrically actuated flue dampers on the market intended for reducing the heat loss from such furnaces caused by unrestricted convective transport of heated interior air up the chimney or flue during those periods when the furnace is not in operation. Obviously, the damper must be open while the furnace is firing. Usually this is done by having the closure of the room thermostat call for the damper to open, and only after such opening has been verified by the closure of an interlock switch is the furnace gas valve or oil burner motor energized.
A number of furnaces now in the field are fitted with manual actuators on their gas valves to permit emergency operation of the furnaces during power outages when solenoid actuation of the gas valves is impossible. The design of the dampers now in the market are required to be such that the damper will be open when such furnaces are under manual control. In a number of arrangements the damper is held shut by electric power and is self-opening by stored energy in biasing means when the power goes off. Of course this will cover normal usuage of the manual operation option of such a furnace. However, there still remains the possibility that someone will manually force the furnace into operation even while power is available which would keep the damper closed. Obviously, this would be a dangerous situation since combustion gases, possibly toxic in nature, would be prevented for the most part from going up the chimney and would be released into the house. Tentative or now existing standards regarding automatic furnace dampers deal with this potential danger by requiring either that the furnace be fitted with a redundant gas valve which cannot be manually actuated, or that the damper itself respond to the presence of hot combustion products to open without a thermostat command. The first option, that of installing a second main gas valve and including its actuation into the normal furnace operation, is considered prohibitively expensive in a retrofit situation. Thus, of those electrically actuated dampers with which I am familiar, and which are intended for use in the retrofit market, all incorporate a normally-closed thermostat in the flue in series with the motor or solenoid which holds the damper closed against the damper opening biasing means.
Such flue thermostats, which typically are of the type commonly used for over heat limit purposes in conventional furnace control systems, consist of one portion such as a helical bi-metal which responds mechanically to the temperature change in such a way that it operates a set of discrete electrical contacts. By the nature of their construction and the quantity of material required, they have some minimum cost no matter how much the electrical rating is reduced. It would be desirable in my estimation if a control system were devised which could eliminate the mechanical flue thermostat.
In considering the possibility of devising a control system in which the mechanical flue thermostat can be eliminated, another consideration enters into the system in connection with the fact that the damper actuator must be energized when the room thermostat does not supply power, and be deenergized when the room thermostat does supply power. As a result, in such systems there is incorporated an electrical inversion device which, conventionally, has been a relay. Relays offer the circuit inversion nicely as well as power gains if required. However, relays are again a manufactured device of many discrete parts so again there is a minimum cost involved no matter how small the electrical rating. Accordingly, in my estimation it would be considered desirable if a control system could be devised which eliminated the relay without a concomitant penalty to the system as a whole because of the inclusion of other similar cost devices to carry out the equivalent function of the relay.
It is the aim of the invention to provide a control system for a flue damper assembly which eliminates the mechanical flue thermostat and relay and utilizes other devices to accomplish the equivalent functions and at less cost for the control system.